[rollei_list] Re: Completely OT- Loudspeaker info
- From: Richard Knoppow <dickburk@xxxxxxxxxxxxx>
- To: rollei_list@xxxxxxxxxxxxx
- Date: Wed, 16 Feb 2005 03:55:46 -0800 (GMT-08:00)
-----Original Message-----
From: Frank Dernie <Frank.Dernie@xxxxxxxxxxxxxx>
Sent: Feb 16, 2005 12:09 AM
To: rollei_list@xxxxxxxxxxxxx
Subject: [rollei_list] Re: Completely OT- Loudspeaker info
It is interesting to study the audibility of phase distortion. I have
listened to a square wave through a loudspeaker and can barely tell
the difference caused by a level of phase distortion which makes the
wave visibly unrecognisable as a square wave on an oscilloscope. One's
eyes are much more phase sensitive than ones ears! In order to do this
one must find a phase coherent loudspeaker and listen in the nearfield,
before the room interactions have had their effect. Very few speakers
on the market are phase coherent.
If phase accuracy was extremely important multi miked recordings mixed
using a conventional desk would sound even worse than they do!
An electrostatic is fairly phase coherent in the first 3 to 4 feet in
front of the speaker. By the time the back reflection is heard (and the
extra stereo "depth" thus created enjoyed) there is no phase coherence
whatever.
A friend of mine designs dipole speakers which sound great. I have some
of his prototype units at home just now. The biggest benefit is the
cost savings associated with the box. Producing an effective closed box
speaker where a considerable proportion of the sound being heard is not
cabinet vibrations is spectacularly expensive, and rarely achieved.
Creating an effective baffle to effectively separate front and rear
waves needs a good understanding of the physics but then can be done
relatively inexpensively. They have to be positioned very precisely
within the room to achieve bass evenness and extension.
The Celestion 6000 system was AFAIK the first to exploit this acoustic
characteristic, the bass module was positioned and orientated
correctly then the main speaker put on top. The position of the bass
units was calculated for each customer by Celestion using the listening
room plan. They are spectacularly effective when thus positioned, but
probably had a tiny market because, as you so rightly write, most
people have to put their speakers where they go!
Frank
On 16 Feb, 2005, at 06:23, A. Lal wrote:
> The ear's sensitivty to group delay (or waveform integrity as you put
> it) is well beyond what most xovers produce.
Keep in mind that multiple microphone technique often uses microphones
rather close to the source so that a fairly isolated input is gotten. I mean
the microphone is not hearing multiple instruments.
The criteria for the mechanical structure of loudspeaker cabinets is often
misunderstood. The idea is to make it high stiffness with fairly low mass and
break up the surface into small sections. That way the resonance of each
section will be well above the bass range and they will become very inefficient
radiators. By using constrained layer damping the radiation efficiency will be
further lowered without creating a heavy structure. Cabinets which contain the
back wave well are not too difficult to build if the principles are understood.
Similar structures are needed for horns or labrynth speakers.
There is a very complex relationship between loudspeaker efficiency, bass
limit, box size, etc. Loudspeakers can be easily made which are too efficient
for use in closed boxes or even bass-reflex enclosures. They will sound weak on
bass unless equalized. A horn will increase the acoustical loading to the point
where the electro-mechanical efficiency can be useful but such horns become
very large for low frequencies and can have other problems such as long path
length and attendant difficulty in matching the wavefront at the crossover
point.
Small box loudspeakers acheive good bass response by throwing away most of
the amplifier power. The speaker is to some extent equalized by making use of
its fundamental resonance. Because such a system must have very large
displacment if the cone size is to be reasonable various tricks are used to
make the suspension linear. Both Paul Klipsch and AR (drawn a blank on a name
I know well) use the trick of using a very low resonance speaker in a small
enclosure such that the stiffness of the enclosed air brought the resonance up
and at the same time acted as a very linear spring. If both sides of a speaker
radiate into the sound space it must be capable of very great cone excursion.
This is difficult to achieve because one must deal with both mechanical
non-linearity and magnetic non-linearity. Small boxes just don't give one
extended bass without paying a price.
A good 15 inch woofer in about a 6 to 8 cubic foot bass reflex cabinet of
proper design will radiate clean bass down to around 35hz with moderate
efficiency. My own experience with stereo reproduction is that uniformity of
distribution at mid range is very important. Also, elimination of delay
differences between woofer and tweeter are very important and will influence
the perception of source size of even a single channel. I have proved this in
listening tests were variable delay was introduced.
As far as crossover networks are concerned, the only simple network which
allows a wave front from multiple speakers to duplicate a single source is a
simple 6db/octave network. This is not steep enough for any but the lowest
power systems. 12db networks are the old standard and are quite satisfactory.
At 18db and greater something additional must be done or the speaker will sound
pinched up. Such steep networks are desirable in high power systems.
All this is so far off topic that I feel guilty about even posting it but I
did a lot of investigating and listening when I was employed in this area some
years ago. I found that the "magic" speakers might sound impressive but that
they did not sound like the original sound sources.
BTW I doubt if any such thing as a truely coherent loudspeaker exists. Oh,
well.
--
Richard Knoppow
dickburk@xxxxxxxxxxxxx
Los Angeles, CA, USA
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